1. Field of the Invention
The present invention relates to deployment of Voice over IP (VoIP) networks, more particularly to interfacing trunk groups between VoIP gateways and Public Switched Telephone Network (PSTN) switches while minimizing dual seizure (i.e., glare).
2. Description of the Related Art
Trunk groups between PSTN switches are managed as a set of circuits, each circuit being identified by a Circuit Identification Code (CIC) in ISUP signaling messages. Two-way trunk groups encounter a problem known as glare or dual seizure. In particular, glare occurs where two switches at each end of a given circuit perform their respective circuit selections for a trunk group, and the outcome is that both switches have selected the same circuit.
Once glare between two switches is detected on a circuit, one of the two switches must be allowed to proceed with its call on the circuit, and the other switch must select an alternate circuit and reattempt its corresponding call on the alternative circuit. The action performed by a switch detecting glare depends on whether the switch controls the circuit. For example, various circuit-selection algorithms are defined, e.g. by the International Telecommunications Union Telecommunication Standardization Sector (ITU-T) Q.764 and by the specification Telcordia GR-317-CORE, to minimize glare or dual seizure between two switches.
One option for glare arbitration, specified in Q.764, involves a switch's corresponding assigned point code, where a switch having a higher point code relative to the peer switch controls all the circuits having even-number CICs, and the switch having the lower point code controls all the circuits having odd-number CICs. Another option is to configure one switch to control all even-numbered circuits, and the other switch to control all odd-numbered circuits; still another option is to configure one switch to control all circuits and configure the other switch to control no circuits.
Minimizing glare becomes more problematic for telephony networks implemented using Voice over IP technology, for example H.323 based networks or Session Initiation Protocol (SIP) based networks, because call processing operations are distributed between different IP-based network nodes, as opposed to conventional time-division switches such as 5ESS switch which utilize centralized switching logic.
FIG. 1A is a diagram illustrating an architecture of a 5ESS switch 10, including multiple interface modules 12 having trunk groups 14, the interface modules 12 connected via a time-multiplexed switch 16 that centrally executes the switching logic. A duplex administrative module processor 18 provides centralized routing control and administrative maintenance features: the administrative module processor 18 communicates with the time-multiplexed switch 16 via a message switch 20. Each trunk group 14 of the 5ESS switch 10 may include large numbers of circuits, on the order of 1,000–4000 circuits. Each interface module 12 includes a duplex time-slot interchange (TSI) 22 used for time-division switching (e.g., utilizing 512 time slots), and multiple interface units 24; each time-slot interchange 22 is connected to the time-multiplexed switch 16 using 32 Mbps network control and timing links for transfer of messages between the administrative module processor 18 and the interface modules 12, as well as for voice transmission.
Hence, the centralized switching logic implemented by the time-multiplexed switch 16 enables circuit selection from the entire set 30 of connected trunk groups 30. The centralized switching logic used by time-division switches such as the 5ESS switch, however, requires deployment of substantially large central office facilities that may not be practical for enterprises (e.g., businesses, universities, etc.) that are attempting to implement their own private telephony network.
Voice over IP based systems often are used to provide an economic and scaleable deployment of a telephony-based system using lower cost components as an alternative to time-multiplexed based switches. FIG. 1B is a diagram illustrating interaction between a Voice over IP based system 32 and a conventional time-division switch such as the 5ESS switch 10. In particular, the Voice over IP based system 32, illustrated for example as an H.323 protocol system, includes a gatekeeper 34 and multiple media gateways 36. The gatekeeper 34 is configured for providing admission control, resource management, security, and routing functionality. In particular, the gatekeeper is configured for routing calls to a selected destination gateway 36 (e.g., 36a) via an IP connection 38 based on either static information specified in routing tables internal to the gatekeeper 34, dial plan tables, etc. in the gatekeeper 34, and/or based on the availability of circuits within the respective gateways 36. In particular, each gateway 36 sends to the gatekeeper 34 via the corresponding IP connection 38 Resource Availability Information (RAI) messages (e.g., according to H.323 protocol) that specify service status; hence, if the gatekeeper 34 determines that the circuits of the gateway 36a are unavailable, the gatekeeper 34 may route calls to another gateway (e.g., 36b).
Each gateway 36 is configured for media conversion of RTP data streams, call establishment and release, circuit selection, and number analysis/translation. In particular, each gateway 36 is configured for managing its trunk groups 40 based on internal execution of the circuit selection algorithms described above; hence, each gateway 36 is configured for executing circuit selection for its corresponding set 42 of connected trunk groups 40 independent of the other gateways 36, providing scalability for increasing the number of trunk groups by adding additional gateways 36. Typically each gateway 36 is configured for supporting up to a maximum of 480 circuits with sixteen E1 links, where groups of circuits are assigned to different trunk groups 40.
The gatekeeper 34 is configured for carrier sensitive routing, where the gatekeeper 34 can route to a gateway trunk group 40 configured on a single gateway 36. This arrangement solves the problem of being able to route to a specific PSTN switch 10 which has a large set 30 of connected trunk groups, since multiple gateways 36 can be configured to have trunk groups that interface with the PSTN set 30 of connected trunk groups 14.
However, a problem arises in that the PSTN switch 10 treats the set 30 of connected trunk groups 14 as a single unit during circuit selection within the trunk group set 30, whereas each gateway 36 performs circuit selection on its own corresponding set 42 of connected trunk groups 40 independently from the sets 42 of trunk groups connected on other gateways 36. Hence, there is a concern of glare on both-way trunk groups deployed using the multiple gateways 36.